To determine if there are sex and ethnic differences in FFA flux and to ensure adequate enrollment of both African American and whites, this investigation is a collaborative effort between the National Institutes of Health (NIH) in Bethesda, Maryland and the Mayo Clinic in Olmsted County, Minnesota. The greater Washington, DC, metropolitan area, where NIH is located, has a large African-American community whereas over 90% of residents of Olmsted County are whites. Therefore, sex differences in FFA flux in African Americans will be mainly studied at the National Institutes of Health. Sex differences in FFA flux in whites will be mainly studied at the Mayo Clinic. The results of the two studies will be combined to determine if there are ethnic differences in FFA flux. To ensure results are attributable to ethnic differences (African American versus white) rather than site differences (NIH versus Mayo) some whites will be enrolled at NIH and some African Americans at Mayo. At NIH, 50 African-Americans (25M, 25W) and 14 whites (7M, 7W) will be enrolled. Equal numbers of normal weight, overweight and obese subjects will be recruited. As outpatients, participants will be placed on weight maintaining diets for two weeks and then admitted for a two-day hospital stay. On each in-hospital morning, resting energy expenditure will be measured and stable isotopes infused to measure FFA flux. Stable isotopes are naturally occurring forms of elements, which are safe and non-radioactive. At NIH 7 African-American men, 14 African-American women and 3 white women have completed the protocol and preliminary analyses are underway.